1. Field of the Invention
This invention is related to microwave acoustics, and more particularly, is related to an improved electro-acoustic transducer structure for acoustic surface wave devices.
2. Description of the Prior Art
Well-known acoustic wave circuits include a source of rf signals, a smooth slab-like element or substrate of a material capable of propagating acoustic surface waves, and a load or utilization device. Electro-acoustic transducers are attached or held in close proximity to the substrate to convert the rf energy to surface waves in the material and vice versa. In general, acoustic surface wave device substrates are fabricated from piezoelectric materials. With such substrates, the input and output transducers commonly take the form of interdigitated electrode fingers bonded or held in close proximity to the substrate surface. Such an electrode array, which may also be described as composed of a pair of interleaved combs of conducting teeth, are coupled to the piezoelectric medium and are utilized to launch acoustic surface waves on that medium. Such acoustic surface waves, or Rayleigh waves are produced upon application of an electrical signal to one of the transducers. An electrical signal is then picked up by the other transducer after a delay equivalent to the time taken by the acoustic waves to propagate from one transducer to the other. Prior art patents of which I am aware which are exemplary of the interdigitated electrode structure include the following U.S. Pat. Nos.: 3,600,710; 3,663,899; 3,742,396; 3,748,603; 3,753,164; 3,790,828; 3,803,520; 3,831,044; and 3,516,027. Such structures may be utilized for various different components such as delay lines, amplifiers, attenuators, filters, and couplers, all having the advantage of micro-miniature construction techniques due to the considerably slower travel of acoustic surface waves than that of electromagnetic waves in free space. In the standard interdigitated transducer, the surface wave is generated by the interaction of the electric fields between the fingers and the material itself through the piezoelectric properties of the material, adjacent fingers being separated by half a wavelength. Details of operation are adequately described in some of the prior art patents, as well as in an excellent article by John deKlerk "Elastic Surface Waves," which appeared in PHYSICS TODAY, November, 1972, pages 32 through 39.
In the deKlerk article, it is explained that the piezoelectric properties of a material are defined by the piezoelectric matrix for that material, the matrix being a 3 .times. 6 array of the form:
d.sub.11 d.sub.12 d.sub.13 : d.sub.14 d.sub.15 d.sub.16 : : (d.sub.ij) = d.sub.21 d.sub.22 d.sub.23 : d.sub.24 d.sub.25 d.sub.26 : : d.sub.31 d.sub.32 d.sub.33 : d.sub.34 d.sub.35 d.sub.36 compression shear
The coefficients d.sub.ij (1.ltoreq.j.ltoreq.3) represent proportionality constants between the compressional stress and electric field components. The coefficients with 4.ltoreq.j.ltoreq.6 represent the shear stresses produced by electric fields. The index i refers to the crystallographic axes. The interdigital transducer of the prior art requires that there be non-zero off diagonal components in the shear half of the piezoelectric matrix in order for the surface waves to be generated.
However, due to the symmetry of many materials, all of the off-diagonal components in the shear portion of the matrix are zero, even though the material is piezoelectric. Zinc blende is one example of such a material. The only way one may utilize such materials with an interdigital transducer is to rotate the crystal so that the transducer is not aligned with one of the principal crystallographic axes. This rotation causes effective off-diagonal piezoelectric coefficients to appear. Some of these coefficients may then be utilized to generate surface waves. A problem with the latter approach is that excessive energy may be dissipated by generating unwanted acoustic waves due to inadvertent coupling through other effective coefficients that have appeared as a result of the rotation of the crystal.
It would therefore be extremely advantageous if a Rayleigh wave generator could be developed with a view towards utilizing those materials with simply non-zero diagonal components. This would open up a whole new class of heretofore unutilized materials for use as acoustic surface wave transducers.